feat: enable custom layouts for error pages (Fixes #755)

[AmanKashyap0807]

Fixes #755

Description

This PR introduces a standardized way to render HTML error pages (404, 500, etc.) using the application's main layout. This resolves the issue where error pages were "naked" (missing headers/footers), making it difficult to include mandatory legal links (Imprint/Privacy) on error screens.

Changes

1. Configuration: Added Genie.config.error_layout (defaults to nothing for backward compatibility).
2. Html Renderer: Updated serve_error_page to check this configuration.
   • If error_layout is set, it renders the specific error view (e.g., :errors, :error_404) wrapped in the defined layout.
   • If nothing, it falls back to the existing behavior (serving static error files).
3. Documentation: Updated README.md to mention the new configuration option.

How to Test (Integration)

1. In a Genie app, create a layout app/layouts/legal.jl.html containing a footer.
2. Create an error view resources/errors/views/error_404.jl.html.
3. Set Genie.config.error_layout = :legal in config/initializers.jl.
4. Trigger a 404 (e.g., visit a non-existent route).
5. Result: The error message renders inside the legal layout.

Verification of Implementation (Unit Simulation)

I verified this locally using a standalone script that mocks the view/layout registration and asserts that the Router produces HTML containing both the layout footer and the error message.

Click to see the verification script used

# BYPASS PKG ACTIVATION TO AVOID VERSION STRING ERROR
push!(LOAD_PATH, @__DIR__)

using Genie
using Genie.Router
using Genie.Renderer.Html

println("Starting verification...")

# --- 1. SETUP: Mock Layout ---
module Layouts
using Genie.Renderer.Html
function legal_layout(; context=@__MODULE__, kwargs...)
    """
    <footer>
        <a href="/imprint">IMPRINT LINK</a>
    </footer>
    <div class="content">$(Genie.Renderer.Html.view!())</div>
    """
end
end
Core.eval(Genie.Renderer.Html, :(legal_layout = $(Layouts.legal_layout)))

# --- 2. SETUP: Mock View Detection for (Resource, ViewName) ---

module MockRenderer
using Genie.Renderer.Html
using Genie.Renderer

# This function mimics the target behavior: rendering the view content
function mock_render(resource::Symbol, view::Symbol; layout=nothing, kwargs...)
    if resource == :errors && view == :error_404
        content = "<h1>Custom 404 Page</h1>"
        # Apply layout if present
        if layout == :legal_layout
            Genie.Renderer.Html.view!(content)
            return Genie.Renderer.Html.legal_layout()
        end
        return content
    end
    return "MOCK_MISS_MATCH"
end
end

# CRITICAL: Mocking the method to capture the specific call signature
Core.eval(Genie.Renderer.Html, quote
    function html(resource::Symbol, view::Symbol; layout=nothing, kwargs...)
        $(MockRenderer.mock_render)(resource, view; layout=layout, kwargs...)
    end
end)


# --- 3. CONFIGURE ---
try
    Genie.config.error_layout = :legal_layout
    println("✅ Configuration accepted error_layout.")
catch e
    println("❌ FAILED: Genie.config.error_layout does not exist.")
    exit(1)
end


# --- 4. EXECUTE ---
println("Attempting to render 404...")

try
    global response = Genie.Router.error("Msg", MIME"text/html", Val(404))

    body_str = String(response.body)
    println("\n--- RESPONSE BODY ---")
    println(body_str)
    println("---------------------\n")

    if occursin("IMPRINT LINK", body_str) && occursin("Custom 404 Page", body_str)
        println("✅ Success: The code correctly calls the layout-aware renderer.")
    else
        println("❌ Failure: Content mismatch.")
    end

catch e
    println("❌ FAILED: Runtime error.")
    showerror(stdout, e)
end

[andreeco]

I tried out something similar to this and it worked. However, I don't know if we should fix the error view files to app/resources/errors/views, especially in regard to Stipple. We should leave it up to the maintainers to decide the best way to handle this. Thanks for your efforts to solve this issue!

julia> julia> Genie.Generator.newapp_mvc("TestApp");

julia> mkpath("app/resources/errors/views")
"app/resources/errors/views"

julia> open("app/resources/errors/views/error_404.jl.html", "w") do f
           write(f, "<h1>Custom 404</h1><p>\$(vars(:message))</p>")
       end
43

julia> open("app/resources/errors/views/error_500.jl.html", "w") do f
           write(f, "<h1>Custom 500</h1><p>\$(vars(:message))</p><span>\$(vars(:info))</span>")
       end
70

julia> Genie.config.error_layout = :app
:app

julia> res404 = Genie.Router.error("Page Not Found", MIME"text/html", Val(404))
HTTP.Messages.Response:
"""
HTTP/1.1 404 Not Found
Content-Type: text/html; charset=utf-8

<!DOCTYPE html><html lang="en">
  <head>
    <meta charset="utf-8" />
    <title>Genie :: The Highly Productive Julia Web Framework</title>
  </head>
  <body>
    <h1>Custom 404</h1>
    <p>Page Not Found</p>
  </body></html>"""

julia> res505 = Genie.Router.error("Crash!", MIME"text/html", Val(500), error_info="Stacktrace info")
HTTP.Messages.Response:
"""
HTTP/1.1 500 Internal Server Error
Content-Type: text/html; charset=utf-8

<!DOCTYPE html><html lang="en">
  <head>
    <meta charset="utf-8" />
    <title>Genie :: The Highly Productive Julia Web Framework</title>
  </head>
  <body>
    <h1>Custom 500</h1>
    <p>Crash!</p>
    <span>Stacktrace info</span>
  </body></html>"""

[AmanKashyap0807]

Thanks for the feedback and for testing it out! @andreeco

You raise a valid point about the directory structure (app/resources/errors/views).

I decided to treat "Errors" like a standard Genie Resource (similar to how we have resources/users or resources/posts) to keep the folder structure consistent with the rest of the framework's MVC pattern.

However, since this is a new feature, I am totally open to moving these files to a different location (like app/views/errors) if the maintainers prefer that standard!